Yet some immunization efforts carry the very rare risk of causing polioviruses to circulate in areas where many people might not yet be vaccinated or areas that were poorly vaccinated — an event that could lead to new cases of disease while trying to demolish it.

It turns out that live strains of poliovirus that are used in the oral poliovirus vaccine can mutate, spread and, in rare cases, even trigger an outbreak, representing a catch-22 in ongoing polio eradication endeavors.

These vaccine-derived viruses continue to haunt certain regions of the world.